1. Field of Invention
The present invention relates to a switching regulator, and a control circuit and method for controlling a switching regulator. Particularly, it relates to such switching regulator, control circuit, and control method which suppresses noises around valleys of a compensation signal.
2. Description of Related Art
FIG. 1A shows a schematic diagram of a typical buck switching regulator 1. As shown in FIG. 1A, the buck switching regulator 1 includes a power stage 11 and a control circuit 13. The power stage 11 switches a switch Q according to a driving signal to convert an input voltage Vin to an output voltage Vout. The control circuit 13 generates the driving signal according to a feedback signal FB. Referring to FIG. 1B, the control circuit 13 includes a driver gate circuit 131, an error amplifier circuit 133 and a comparator 135. The error amplifier circuit 133 compares the feedback signal with a reference signal Vref, and generates a compensation signal Vc according to the comparison result. As shown in FIG. 1A, the feedback signal FB is obtained from the voltage across a resistor R2, which is a divided voltage by resistors R1 and R2 connected in series between the output voltage Vout and ground level. The comparator 135 compares the compensation signal Vc with a current sense signal CS which is related to an inductor current IL in the power stage 11, and the comparator 135 generates a pulse width modulation (PWM) signal according to the comparison result. The PWM signal is converted to a driving signal by the driver gate circuit 131, to operate the power switch Q. The power stage 11 can be one of the synchronous and asynchronous buck, boost, inverting, buck-boost, and inverting-boost conversion circuits shown in FIGS. 2A-2J.
Referring to FIG. 1C, wherein the buck switching regulator 1 as shown in FIG. 1A is taken as an example, when the input voltage Vin is a rectified AC signal, the ideal waveform of the input voltage Vin is a semi-sinusoidal voltage signal as shown on top of FIG. 1C; but in an actual condition, the waveform of the input voltage Vin is as the second waveform shown in FIG. 1C, referred to as “valley-cut semi-sinusoidal voltage signal” in this invention, wherein the lowest level of the input voltage Vin is not lower than the output voltage Vout. Therefore, corresponding waveforms of the inductor current IL and the compensation signal Vc are as shown by the third and fourth waveforms, respectively. When the input voltage Vin is not higher than the output voltage Vout, because the feedback signal FB or the current sense signal CS is equal to or near zero, and due to the reverse amplification effect of the error amplifier circuit 133, the waveform of the compensation signal Vc is distorted with noises around valleys of the compensation signal Vc (as shown in the figure, the noises are indicated by dashed circles), adversely impacting the power factor (PF) and the total harmonic distortion (THD).
Besides, in the applications of driving a light emitting diode (LED) circuit wherein a tri-electrode AC switch (TRIAC) is used, because the TRIAC needs to be triggered, the trigger action also enhances the noises around valleys of the compensation signal Vc. Furthermore, in the applications of frequency modulated switching regulators, the frequency should be limited under a certain level lest the inductor current IL will not decrease, so the compensation signal Vc needs to be reset at proper timings to limit the operation frequency. In other types of power conversion circuits, noises often also exist around the valleys of the compensation signal Vc, or it is required to periodically adjust or reset the compensation signal Vc.
In view of the foregoing, the present invention provides a switching regulator, and a control circuit and method for controlling a switching regulator, which is capable of adjusting the compensation signal Vc to suppress the noises around the valleys of the compensation signal Vc, such that the power factor PF and the total harmonic distortion THD are improved.